Channelized filter using semiconductor fabrication

ABSTRACT

A semiconductor technology implemented high-frequency channelized filter includes a dielectric substrate with metal traces disposed on one of two major surfaces of the substrate. An input and output port disposed on the substrate and one of the metal traces carrying a high-frequency signal to be filtered between the input and output port. Other of the metal traces are connected to the one metal trace at intervals along the length of the one metal trace each providing a reactance to the high-frequency signal where the reactance varies with frequency and additional traces of the metal traces serving as a reference ground for the one metal trace and the other metal traces. A silicon enclosure mounted to the substrate with a first planar surface with cavities in the enclosure that extend through the first surface, and internal walls within the silicon enclosure defining the cavities. A layer of conductive metal covers the first planar surface, cavities and the internal walls. The silicon enclosure having substantially continuous areas of metal on the first planar surface about the periphery of the silicon enclosure that engage corresponding areas of the additional traces about the periphery of the substrate. The cavities surround the respective other metal traces with the internal cavity walls engaging the additional traces adjacent the respective other metal traces to individually surround each of the other metal traces with a conductive metal thereby providing electromagnetic field isolation between each of the other metal traces.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S. patentapplication Ser. No. 16/916,644, filed Jul. 17, 2020, entitled“CHANNELIZED FILTER USING SEMICONDUCTOR FABRICATION” the entire contentsof which are incorporated herein by reference.

BACKGROUND

Embodiments of the invention relate to channelized microstrip filtersmade using semiconductor fabrication technology with an enclosurecomposed of micromachined interiors that enhance the performance of themicrostrip filters and provide manufacturability that yields repeatableperformance results.

High-frequency, i.e. frequencies of 1 GHz and higher, microstrip filtershave been constructed using a variety of materials and techniques. It ispreferable to test subassemblies prior to being installed in a largercircuit assembly so that the assembly is built of known good devices.However, the ability to test a high-frequency modular microstrip filterto confirm that it has acceptable performance prior to installation inthe larger assembly has proven to be a substantial challenge, in partbecause the filter performance is sensitive to the housing in which itis assembled. If it is discovered that the modular microstrip filterdoes not meet performance specifications after being installed as partof the larger assembly, the modular microstrip filter may require atedious in-place tuning process or may require removal and replacementwith another modular microstrip in order to yield acceptable filterperformance in the larger assembly.

In order to provide electromagnetic isolation an RF filter, a housing,such as of sheet metal, machined metal, or a casted metal, may be usedto enclose the microstrip filter. However, variations in the physicaldimensions of the metal housing often results in undesired variations inelectromagnetic coupling within the filter elements and leads tovariations in the performance of the filter.

There is a desire to minimize the area occupied by high-frequencyfilters as part of an overall desire towards miniaturization ofelectronic circuits. Reducing the overall area of the filter requiresthat individual internal elements of the filter be increasingly closertogether. This increases the likelihood of electromagnetic fieldinteraction between the elements which leads to increased challenges inbeing able to account for such interactions during the design of thefilter and provides an undesired variable that adversely impactsreproducible performance from unit to unit.

There exists a need for an improved high-frequency microstrip filterthat minimizes these difficulties.

SUMMARY

It is an object of embodiments of the present invention to provideimproved modular microstrip filters that minimize such difficulties.

A semiconductor technology implemented high-frequency channelized filterincludes a dielectric substrate with metal traces disposed on one of twomajor surfaces of the substrate. An input and output port disposed onthe substrate and one of the metal traces carrying a high-frequencysignal to be filtered between the input and output port. Other of themetal traces are connected to the one metal trace at intervals along thelength of the one metal trace each providing a reactance to thehigh-frequency signal where the reactance varies with frequency andadditional traces of the metal traces serving as a reference ground forthe one metal trace and the other metal traces. A silicon enclosuremounted to the substrate with a first planar surface with cavities inthe enclosure that extend through the first surface, and internal wallswithin the silicon enclosure defining the cavities. A layer ofconductive metal covers the first planar surface, cavities and theinternal walls. The silicon enclosure having substantially continuousareas of metal on the first planar surface about the periphery of thesilicon enclosure that engage corresponding areas of the additionaltraces about the periphery of the substrate. The cavities surround therespective other metal traces with the internal cavity walls engagingthe additional traces adjacent the respective other metal traces toindividually surround each of the other metal traces with a conductivemetal thereby providing electromagnetic field isolation between each ofthe other metal traces. Conductive engagement is formed between thefirst planar surfaces and the additional traces to establish a commonreference ground therebetween.

A high-frequency filter having a substantially planar dielectricsubstrate and metal traces that are lithographically fabricated on oneof two major surfaces of the substrate is also provided in which aninput and output port are disposed on the substrate. One of the metaltraces carrying a high-frequency signal is to be filtered between theinput and output port. Other of the metal traces, connected to the onemetal trace at intervals along the length of the one metal trace, eachproviding a reactance to the high-frequency signal where the reactancevaries with frequency. Additional traces of the metal traces serve as areference ground for the one metal trace and the other metal traces. Amicro-machined silicon enclosure is mounted to the substrate with afirst planar surface with cavities in the enclosure that extend throughthe first surface, and internal walls within the silicon enclosuredefine the cavities. A layer of conductive metal covers the first planarsurface, cavities and internal walls. The silicon enclosure hassubstantially continuous areas of metal on the first planar surfaceabout the periphery of the silicon enclosure that engage correspondingareas of the additional traces about the periphery of the substrate.Each cavity surround at least one of the other metal traces with theinternal cavity walls engaging the additional traces adjacent therespective other metal traces to individually surround the at least oneof the other metal traces with a conductive metal thereby providingelectromagnetic field isolation between cavities. Conductive engagementis formed between the first planar surfaces and the additional traces toestablish a common reference ground therebetween.

DESCRIPTION OF THE DRAWINGS

Features of exemplary embodiments of the invention will become apparentfrom the description, the claims, and the accompanying drawings inwhich:

FIG. 1 shows a perspective view of a high-frequency channelized filterin accordance with an embodiment of the present invention with anenclosure in an open position;

FIG. 2 shows a perspective view of the channelized filter as shown inFIG. 1 with the enclosure in final assembled position;

FIG. 3 shows a representative cross-section of the assembled channelizedfilter in accordance with the embodiment of the present invention;

FIG. 4 shows a graph illustrating performance characteristics of theexemplary channelized filter over a frequency range in accordance withan embodiment of the present invention;

FIG. 5 is a partial perspective view of an alternative embodiment of achannelized filter of the present invention that shows the ability totest the performance of the filter from the bottom surface of the planarsubstrate;

FIG. 6 shows a partial, exploded top view of the embodiment of FIG. 5.

DETAILED DESCRIPTION

One aspect of the present invention resides in the recognition of thedifficulties associated with repeatably manufacturing high-frequencychannelized filters with consistent performance that does not requirepost-manufacture tuning by minimizing cross-coupling of electromagneticfields between filter elements. Effective element shielding is importantto minimize such cross-coupling, especially between adjacent elements,in order to minimize undesired unit to unit performance variations.Effective element shielding also allows filter elements to be compactedby using meandering filter elements to minimize the total area footprintof the filter.

Another aspect of the present invention resides in the recognition of animprovement in input/output coupling that facilitates the ability toreliably test the performance of a high-frequency modular filter priorto installation of the filter in a larger electronic assembly. In oneembodiment, testing from ports on the bottom surface of the substrateopposite to that containing the filter elements provides ease of accessfor test probes and for connection in a larger circuit assembly.

FIG. 1 shows a perspective view of a high-frequency channelized filter100, e.g. a microstrip filter, in accordance with an embodiment of thepresent invention with a substrate 105 and an enclosure 110 in an openposition. The term “channelized” is used herein to refer to the use ofconductive channels that separate individual elements of the filter asopposed to a single enclosure space that covers multiple elements or anentire filter/circuit. A primary signal conductor (transmission line)115 is disposed on the top side of substrate 105, which is preferablymade of a low-loss dielectric such as silicon carbide, alumina, InP,GaAs, quartz, and extends between input and output ports, 120, 125. Areference conductor 130, which acts as ground, extends across variousregions on the top of substrate 105 and is interconnected by a pluralityof vias 135 to a reference conductor/ground 140 located acrosssubstantially the entire bottom side of substrate 105. A plurality ofindividual filter elements 145, 150, 155, and 160 are selected toprovide frequency varying values of inductance and/or capacitance to therespective points of connection to the primary signal conductor 115. Thecombined effect of the selected inductance and capacitance values of thefilter elements as disposed at locations along the signal line definethe passband and rejection characteristics of the illustrative bandpassmicrostrip filter.

The enclosure 110, when in the closed position, substantially surroundsthe top peripheral surface of the substrate 105. Enclosure 110 ispreferably made of silicon with a planar surface 165 disposed to engagethe top surface of substrate 105. The enclosure 110 contains a pluralityof cavities 170, preferably formed by deep reactive ion etching (DRIE)for micro-precision dimensions. The silicon micro-machining enablestightly controlled geometries of the electromagnetic cavities 170 of thechannelized microstrip filter. The cavities 170 correspond to thoseareas on the top surface of substrate 105 that will help individuallyencapsulate the filter elements and the signal line. The surfaceportions of planar surface 165 are conductive, i.e. preferably platedwith a good conductor such as gold, and are disposed to engage thereference/ground areas 130. The vertical sidewalls 175 and the bottom180 of the cavities formed in the cover are also conductive, i.e. alsopreferably plated with a good conductor such as gold. Thus, when theenclosure 110 is placed in the assembled position to engage the topsurface of substrate 105, all surfaces of the enclosure facing thesubstrate are continuously conductive and connected to the ground 130.The ends of the interior walls formed by the etched cavities 170 of theenclosure 110 correspond to and are disposed to engage correspondingground areas 130, including those ground areas interior of the peripherythat separate the individual filter elements. Therefore, each of theindividual filter elements are enclosed above the substrate within aseparate volume/chamber that is grounded which provides isolation andbasically eliminates undesired cross-coupling between filter elements.Cut-out portions 190 in the end walls of the enclosure 110 provide anopening for access to the input 120 and output 125 signal lines. Thisfacilitates not only connection of the input and output signal line ofthe channelized microstrip filter to a larger circuit assembly but alsoallows for the temporary engagement of probes to test the filter priorto installation in the larger circuit assembly.

FIG. 2 shows a perspective view of the channelized filter 100 with theenclosure 110 in a final assembled position engaging the substrate 105to provide a continuous peripheral ground connection with the ground 130of the substrate except for the cut-out portions 190. Also, each of theinterior walls defined by the etched areas 170 in the enclosure 110engage corresponding ground areas 130 on the substrate 105. As will beseen by referring to FIG. 1, each of the elements 145, 150, 155, 160will be totally surrounded on and above the top of the substrate 105 bya metallic ground surface with only portions of each of the elementsthat form a connection with the signal line 115 extending outside of theindividual metallic enclosures. Because the ground 140 extendssubstantially over the entire bottom side of the substrate 105 and isconnected by the plurality of vias 135 distributed throughout the groundarea to the ground on the top side of the substrate 105, each of thefilter elements is also surrounded on the bottom of the substrate 105 bya metallic ground. The degree of encapsulation of the filter elementsprovides a very effective electromagnetic shield that minimizes crosscoupling between elements as well as preventing coupling to circuitryoutside of the channelized microstrip filter. In order to provide aneffective continuous electromagnetic grounding of the metal 130 on thetop side of the substrate and the metal 140 on the bottom side of thesubstrate at the high-frequencies of operation of the channelizedmicrostrip filter, each of the top and bottom metals are connected by aplurality of vias that preferably have a spacing not more than 0.1wavelengths for the highest frequencies of operation. Preferably, theconductive layers on surface 165 of enclosure 110 that engage conductivemetal 130 on substrate 105 are coupled together using gold-to-goldthermocompression bonding for micron precision assembly.

Exemplary circuit elements are implemented by the traces 145, 150, 155and 160 as shown in FIG. 1. However, those skilled in the art willunderstand that these filter elements are merely exemplary of varioustypes and numbers of filter elements and layouts, and that other filterconfigurations and layouts can be used to provide desired frequencyselectivity using the techniques of the embodiments of this invention.For example, the conductor 115 in a different filter topology may not becontinuous or a DC short-circuit between the input and the output; itcould be made non-continuous by an element such as “pi-of-cap” thatintroduces gaps consisting of three capacitors in tandem—a shuntcapacitor, a series capacitor, and a second shunt capacitor. Similarly,the short-circuited shunt stubs could be open-circuited shunt stubs,coupled-line stub, or even higher order sub-circuit such as inductivesegment followed by a capacitive segment.

In order to minimize the footprint area occupied by the filter, theindividual filter elements meander over the length of the filterelements. The signal line 115, metallic ground traces 130 on the topsurface of the substrate, and the filter elements reside in a commonplane parallel to the plane of the substrate. The filter elementsmeander within this plane. As used herein, “meander” means to turn atone or more angles, preferably at 45 degrees or more within the sameplane. Using 90 degrees as an example, filter element 160 consists of afirst segment 161 that is connected to the signal line 115, a secondsegment 162 coupled to the end of segment 161 and being perpendicular tosegment 161, and segment 163 coupled to the end of segment 162 and beingperpendicular to segment 162 and parallel to segment 161. In comparison,a conventional filter element would typically extend in a straight linewhich would require a substantially wider and/or longer substrate thanthe substrate 105 and cause the associated substrate of the filter tohave a substantially larger footprint area. A meandering approach toreduce the filter footprint in a conventional “open-face” filter(without channelized filter elements) brings serious challenges. Such anapproach would require substantially increased design effort because theinteractive coupling from one filter element to another filter elementor the signal line would require repeated electromagnetic simulationsand trial-and-error experiments to resolve issues of raised return loss,skewed slope of fall-off, undesired spikes in the stop band rejectionetc. And yet the final design outcome of such an approach is sensitiveto manufacturing tolerance and the housing channel height and width dueto the cross coupling of all elements through an empty space. The groundsegments 131, 132 and 133 are spaced apart from the segments of filterelement 160 and function to surround the entire meandering length of thefilter element. These ground segments in combination with thecorresponding engaged walls of the associated cavities provide aneffective grounded chamber for the entirety of the meandering filterelement 160 except for the small portion of 160 that connects to thesignal line 115.

In order to prevent undesired cross coupling of segment 163 of thefilter element with segment 161, the ground segment 132 is disposedtherebetween which together with the corresponding engaged wall of theassociated cavity provides isolation between these two segments. Groundsegment 131 also serves to provide isolation between segment 161 of thefilter element and a parallel portion of the signal line 115. Groundsegment 133 provides isolation between segment 163 of the filter elementand adjacent filter element 155 which has a portion parallel to segment163. Of course, the walls of the associated cavities that engage theground segments 131 and 133 complete the corresponding chambers thatprovide isolation.

FIG. 3 shows a representative cross-section of the channelized filter100 as assembled with the enclosure 110 engaging the substrate 105. Alayer of gold plating 310 covers the interior surfaces of enclosure 110and surfaces 165 that engage the metallic grounds 130 on the top surfaceof the substrate 105. The enclosure is preferably made of amicromachined silicon wafer on which a layer of metal is depositedhaving a peak to valley roughness of less than 1 microns. Suchsmoothness contributes to consistent performance and reduced lossesespecially at higher frequencies.

Representative vias 135 provide continuity between the top and bottomground metallization on the substrate. In one example of an embodiment,the overall height 320 of the enclosure 110 is approximately 1 mm, theinternal cavity height 325 is approximately 0.635 mm, and the width 330of the cavity is approximately 0.800 mm. These dimensions coincide withthe response of the exemplary filter discussed with regard to FIG. 4.

FIG. 4 is a graph 400 illustrating performance characteristics of theexemplary channelized filter 100 over a frequency range of 0 GHz to 5GHz. The vertical axis represents decibels (dB) and the horizontal axisshows frequency in gigahertz. Curve 405 shows the bandpass filtertransmission characteristics (S₂₁) showing the passband with relativelylow signal loss from about 1.5 GHz to 3.5 GHz with increasing signalloss below and above this range. Curve 410 shows the input reflectionloss (S₁₁) that begins to increase starting about 1 GHz and returns torelatively low values about 4 GHz. Input reflection loss within thebandpass filter range is a minimum of 20 dB. These filtercharacteristics for the exemplary channelized microstrip filter 100 withcompressed/folded/meandering filter elements compare favorably to thecharacteristics for a conventional microstrip filter that occupies asubstantially larger area than filter 100 and uses straight line filterelements.

FIGS. 5 and 6 are a partial perspective views of an alternativeembodiment 500 of a channelized filter that includes bottom access portson the bottom surface 505 of the planar substrate 510 that provide theability to engage probes and test the performance of the filter. Sincethe differentiating features of filter 500 relative to filter 100 relateto input/output ports accessible from the bottom 505 of the substrate510 as opposed to access to the input/output ports disposed on the topside of the substrate for filter 100, only one of two exemplaryinput/output port 515 is shown.

FIG. 5 shows a partial view of the bottom 505 of substrate 510 whileFIG. 6 shows a partial view of the top 520 of substrate 510. Referringto FIG. 5, a section 525 of the bottom 505 is shown removed to show therows of vias connecting the top and bottom surface metallization(ground). The signal line 530 for filter 500 is the same as signal line115 for filter 100. As seen in FIG. 6, signal line 530 terminates atmetallization 535 on the top side 520 of the substrate and is flanked bymetallic ground regions 540. This is the structure as provided for theinput/output ports for filter 100. As seen in FIG. 5, a region 545 onthe bottom 505 of substrate 510 has no metallization. A via 550 connectsa strip of metallization 555 on the bottom 505 with metallization 535disposed on the top surface 520. The via 550 provides a conductivecoupling of the signal on the signal line 530 to the metal conductor 555on the bottom of the substrate. The expanded areas 560 of region 545without metallization on the bottom of the substrate provide animpedance transformation to provide a 50 ohm impedance for the port 515.As seen in FIG. 5, a probe 565 includes a center signal conductor 570and a pair of ground terminations 575 shown engaging one of the inputand output ports 515 on the bottom of the substrate. The other bottomport (not shown) is identical and provides access to the other of theinput and output ports. Ports on the bottom surface of the substratesimplify testing and allow probes (and connection points to othercircuitry in the larger assembly) easy access and coupling withoutrequiring a cutout of the enclosure as in filter 100. Thus, theenclosure of filter 500 does not require and does not have a cutout inthe end walls. This provides increased shielding as there is noopportunity for electromagnetic fields to enter/exit through the cutoutareas and reduces manufacturing complexity of the enclosure. Similarly,there is no opportunity for foreign objects to enter into the filtercavity through the cutouts and cause detrimental effect such asde-tuning and short-circuiting on filter performance. It is understoodthat the shapes 540 are actually connected to the ground andmanufactured as one metallization instead of two separatemetallizations. The difference is that Si cover lands on the ground butnot on 540.

The concept of a channelized filter is not limited to the exemplarymicrostrip line where the signal trace runs on top of a ground plane anda top enclosure forms individual chambers around each filter element. Itis also applicable to other types of transmission lines such asstripline where the signal trace on a substrate is captivated between atop ground enclosure and a bottom ground enclosure, i.e. where a topchannelized enclosure and a mirror image bottom channelized enclosurecooperate to surround and sandwich the individual respective filterelements within individual separate chambers.

Although exemplary implementations of the invention have been depictedand described in detail herein, it will be apparent to those skilled inthe art that various modifications, additions, substitutions, and thelike can be made without departing from the spirit of the invention. Forexample, the cavities can be different heights and various bondingtechniques including eutectic bonding such as indium-gold or gold-tin,or copper pillar bonding could be used to attach the enclosure to theground metallization on the substrate. The enclosures of the channelizedfilters can be bonded to a plurality of corresponding substratesmanufactured on a single wafer rather than as separated substrates. Thecavity height is only limited by the fabrication capability of thesilicon etching tool. A silicon enclosure with two different etch(cavity) depths is possible.

The scope of the invention is defined in the following claims.

We claim:
 1. A semiconductor technology implemented high-frequencychannelized filter comprising: a substantially planar dielectricsubstrate; metal traces disposed on one of two major surfaces of thesubstrate; an input and output port disposed on the substrate; one ofthe metal traces carrying a high-frequency signal to be filtered betweenthe input and output port; a plurality of other of the metal traces,connected to the one metal trace at intervals along the length of theone metal trace, each providing a reactance to the high-frequency signalwhere the reactance varies with frequency; additional traces of themetal traces serving as a reference ground for the one metal trace andthe other metal traces; a silicon enclosure mounted to the substratewith a first planar surface with cavities in the enclosure that extendthrough the first surface, internal walls within the silicon enclosuredefining the cavities; a layer of conductive metal covers the firstplanar surface, cavities and internal walls; the silicon enclosurehaving substantially continuous areas of metal on the first planarsurface about the periphery of the silicon enclosure that engagecorresponding areas of the additional traces about the periphery of thesubstrate; the cavities surround the respective other metal traces withthe internal cavity walls engaging the additional traces adjacent therespective other metal traces to individually surround each of the othermetal traces with a conductive metal thereby providing electromagneticfield isolation between each of the other metal traces; and conductiveengagement is formed between the first planar surfaces and theadditional traces to establish a common reference ground therebetween.2. The filter of claim 1 further comprising corresponding cavitiesdimensioned with associated cavity walls that engage additional tracesadjacent the one metal trace to substantially surround the one metaltrace with a conductive metal between the input and output ports therebyproviding electromagnetic field isolation.
 3. The filter of claim 1wherein the substrate is silicon carbide and the metal is depositedgold.
 4. The filter of claim 1 further comprising: metal deposited onthe other major surface of the substrate; and a plurality of closelyspaced, contiguous, through-hole conductive vias disposed to connect theadditional traces on the one surface of the substrate to the metaldeposited on the other major surface of the substrate to define commonreference ground.
 5. The filter of claim 1 wherein the cavities areformed of a micromachined silicon wafer and the metal having a peak tovalley roughness of less than 2 microns.
 6. The filter of claim 1wherein the input and output ports are disposed on the one of the majorsurfaces of the substrate, and the enclosure having cutaway sections ofa peripheral wall adjacent the input and output ports.
 7. The filter ofclaim 1 wherein at least one of the plurality of other of the metaltraces meanders along its length to minimize the area of the substraterequired to support the other metal traces.
 8. The filter of claim 7wherein at least 50% of the plurality of other of the metal tracesmeander along the respective lengths to minimize the area of thesubstrate required to support the other metal traces.
 9. The filter ofclaim 7 wherein all of the plurality of other of the metal tracesmeander along the respective lengths to minimize the area of thesubstrate required to support the other metal traces.
 10. The filter ofclaim 7 wherein the one of the metal traces meanders between the inputand output ports.
 11. The filter of claim 1 further comprising: a firstvia connecting the one metal trace near one end of the one metal traceon the one of the major surfaces of the substrate to an input metaldisposed on the other of the major surface of the substrate, and asecond via connecting the one metal trace near the other end of the onemetal trace on the one of the major surfaces of the substrate to anoutput metal disposed on the other of the major surface of thesubstrate; and the input and output ports connected to the input metaland output metal, respectively, and are disposed on the other of themajor surfaces of the substrate, thereby facilitating easy access to theinput and output ports for testing performance of the filter prior toinstallation of the filter in a larger circuit assembly.
 12. The filterof claim 11 wherein the silicon enclosure has a contiguous area of thefirst planar surface enclosing the entire periphery of the siliconenclosure that is dimensioned to engage corresponding contiguous areasof the additional traces about the entire periphery of the substrate sothat no openings exist between the interior of the enclosure and theexterior.
 13. The filter of claim 11 further comprising a means forimpedance matching associated with the input and output metals on theother of the two major surfaces of the substrate to transform theimpedance at the input and output metals adjacent the respective vias tosubstantially 50 ohms for testing and coupling to external circuitry.14. A high-frequency filter comprising: a substantially planardielectric substrate; metal traces that are lithographically fabricatedon one of two major surfaces of the substrate; an input and output portdisposed on the substrate; one of the metal traces carrying ahigh-frequency signal to be filtered between the input and output port;a plurality of other of the metal traces, connected to the one metaltrace at intervals along the length of the one metal trace, eachproviding a reactance to the high-frequency signal where the reactancevaries with frequency; additional traces of the metal traces serving asa reference ground for the one metal trace and the other metal traces; amicro-machined silicon enclosure mounted to the substrate with a firstplanar surface with cavities in the enclosure that extend through thefirst surface, internal walls within the silicon enclosure defining thecavities; and a layer of conductive metal covers the first planarsurface, cavities and internal walls; the silicon enclosure havingsubstantially continuous areas of metal on the first planar surfaceabout the periphery of the silicon enclosure that engage correspondingareas of the additional traces about the periphery of the substrate;each cavity surrounding at least one of the other metal traces with theinternal cavity walls engaging the additional traces adjacent therespective other metal traces to individually surround the at least oneof the other metal traces with a conductive metal thereby providingelectromagnetic field isolation between cavities; and conductiveengagement is formed between the first planar surfaces and theadditional traces to establish a common reference ground therebetween.15. The filter of claim 14 wherein the substrate is silicon carbide andthe metal is deposited gold.
 16. The filter of claim 14 furthercomprising: metal deposited on the other major surface of the substrate;and a plurality of closely spaced, contiguous, through-hole conductivevias disposed to connect the additional traces on the one surface of thesubstrate to the metal deposited on the other major surface of thesubstrate to define common reference ground.
 17. The filter of claim 14wherein the cavities are formed of a micromachined silicon wafer and themetal having a peak to valley roughness of less than 2 microns.